It is known to manufacture various styrene polymers, especially styrene homopolymers or copolymers, in particular:                a standard polystyrene (or standard PS), which is generally an amorphous polystyrene homopolymer, of amorphous structure and shiny appearance, and is rigid and transparent, often called “crystal PS” because of its clarity, and which generally has a weight-average molecular weight Mw ranging from 150 000 to 400 000 daltons;        a high-impact polystyrene (or HIPS) which is impact resistant and of opaque appearance, this being a polystyrene modified by grafting onto a rubber or an elastomer, whether natural or synthetic, such as a polybutadiene, more precisely a polybutadiene/styrene copolymer;        a styrene/butadiene (SB) block copolymer or clear-impact polystyrene (CLIPS), which is rich in styrene, impact resistant and transparent in appearance; and        an expandable polystyrene (or PSE) in the form of beads, containing a blowing agent, and intended to manufacture an expanded and moulded polystyrene, of cellular structure, which is rigid and of low density.        
These styrene polymers result from an exothermic styrene polymerization reaction and are generally manufactured by bulk, solution, aqueous suspension or aqueous emulsion polymerization processes. These processes may be carried out continuously or in batch mode, in one or more mechanically stirred polymerization reactors.
In a bulk or solution styrene polymerization process the liquid polymerization reaction mixture becomes increasingly viscous as the polymerization reaction proceeds. The stirring of this mixture is an important element for controlling both heat exchange and quality of the polymer. One of the objectives of the present invention is therefore to provide a stirring device that is especially suitable for this process.
In the case of a process for manufacturing a high-impact polystyrene (HIPS), at least one prepolymerization step is carried out by bringing the styrene into contact with a rubber under conditions that allow both the styrene to be polymerized and the styrene polymer to be grafted onto the rubber. During this step, a phenomenon generally called a phase inversion occurs: the styrene polymer, which is initially insoluble in a continuous phase comprising the styrene and the rubber, becomes a continuous phase in which a new discontinuous phase appears, this essentially consisting of rubber nodules. During the phase inversion, the liquid polymerization mixture undergoes large variations in viscosity. The stirring of the liquid mixture is here too an important element for controlling the quality of the polymer, and especially the size and the size distribution of the rubber nodules included in the polystyrene matrix. One of the objectives of the present invention is to be able to provide a stirring device that is especially suitable for this process, in particular for the purpose of controlling the phase inversion step and the size and size distribution of the rubber nodules, independently of the presence of known additives, especially so as to influence the size of the nodules. One of the objectives is therefore to be able to provide a stirring device intended to prepare a high-impact polystyrene, especially one modified by grafting onto a rubber, and in which the rubber nodules have a relatively large size, for example ranging from 6 to 12 μm, in particular from 8 to 12 μm, with a relatively narrow size distribution.
In the case of a process for polymerizing styrene in aqueous suspension, especially intended for manufacturing expandable polystyrene beads, the stirring of the suspension is also an important element, which makes it possible to keep the polymer beads being formed in suspension and also to control the size and size distribution of the beads, in particular independently of the presence of a suspension stabilizer. One of the objectives of the present invention is therefore to be able to provide a stirring device that is especially suitable for this process.
It is known that these processes may be carried out in a mechanically stirred polymerization reactor, especially in the form of a vertical cylindrical vessel provided with a mechanical stirring device comprising a central rotation shaft and one or more stirring members attached to the shaft. A drive system connected to the central shaft and comprising a motor rotates the shaft and the stirring members. Various stirring members may be chosen depending on the desired movement of the liquid polymerization mixture, this movement being generated by the rotation of the shaft and the stirring members. The hydrodynamic state thus created in the vessel depends in general on the type of stirring member, on the geometrical factors of the vessel, such as the shape and dimensions of the vessel, and on the presence or otherwise of baffles attached to the vessel. It has been observed in particular that if the vessel is not provided with a baffle and if the stirring axis is coincident with the axis of the vessel, the movement of the liquid polymerization mixture has vertical velocity components that may be very small. A vortex may form, with the disadvantage of limiting the dissipating power of the stirring motor and a lack of homogeneity of the liquid polymerization mixture. In such a reactor, at least one baffle can be fitted, either attached to the wall of the vessel or away from the wall. A baffle away from the wall generally consists of a vertical bar having the shape of a vertical cylinder of revolution. However, it has been found that a baffle used in this way in one of these polymerization processes generally results in fouling of the baffle, especially in a deposit of heterogeneous substances on the face of the baffle on the opposite side from the face exposed to the movement of the liquid polymerization mixture. The particularly high viscosity of the liquid mixture and the possible variations in this viscosity over time may result in detachment (or tear-off) of this deposit, which is then not easily dispersed within the liquid mixture and will disturb the execution of the polymerization reaction. The result, the quality of the polymer is seriously affected. One of the objectives of the present invention is therefore to provide a stirring device which includes in particular at least one baffle, making it possible to reduce or even prevent fouling of the baffle and to manufacture a styrene (co)polymer whose quality is satisfactory and stable over time.
In the case of a process for polymerizing styrene in aqueous suspension, it has also been observed that, depending on the stirring member and the optional presence of a baffle, the radial velocity (especially the tangential or peripheral velocity) of the liquid polymerization mixture may become excessive and lead to loss or destabilization of the suspension, despite the presence of a suspension stabilizer. It may also result in the liquid polymerization mixture being stirred with a higher axial velocity, in such a way that an intake phenomenon may occur, during which part of the gaseous phase lying above the liquid phase is taken into the suspension, and finally a loss or destabilization of the suspension. One of the objectives of the present invention is therefore to provide a stirring device that allows the liquid polymerization mixture to be kept stable during aqueous suspension polymerization of the styrene.